RESPIRATION, ORGANS AND PROCESS OF, The great objects of respiration or breath ing are, first, the introduction into the system of oxygen, by which the products result ing from the disintegration or breaking up of the muscular, nervous, and other tissues of the converted into compounds, which are easily eliminated or removed by the excreting organs (as the kidneys, lungs, skin, etc.); and, secondly, the removal of the most noxious, and consequently, the most important of these products, carbonic acid, through special respiratory organs, which, in most air-breathing animals, except insects. are lungs; while in water-breathing animals, excepting those very low in the scale of organization, they take the form of bra achbe, or gills. In all the vertetrated animals, excepting in fishes. and in the amphibians during their young state,* the respiratory organs are more or less complicated internal alr-sacs, communicating through the throat with the external atmosphere. The simplest known form in which these LUNGS or inter nal air-sacs exist is as a pair of elastic membranous bags placed close beneath the vertebral column, communicating with the surrounding atmosphere by a tube known as the wind pipe, or trachea, which opens through the larynx, or organ of voice, into the throat. These bags are lined by a delicate, thin, and moist membrane, calleda innepus membrane, embedded in, and partly beneath which is a vascular network, through which all the bloat] in the animal's body is in turn driven by the heart. The moist partition between the blood in this network and the air in the interior of the lungs is so thin, that after having its moisture) dissolved the oxygen of the air, it permits of its passage into the moving current of blood, whilst through the same agencies carbonic acid simultaneously passes in an opposite direction from the blood into the air. To complete the apparatus, there are certain muscles under whose action the bags are emptied of their vitiated con tents and refilled with pure air. Such are the respiratory organs as they occur in that remarkable animal, the proteus anguinus, foun d in the dark caves of Carinthia, and belong ing to the order amphigneusta, referred to in the foot-note. In the mere highly organ ized animals and in man, we find these elementary essential parts complicated and modi fied in a great variety of ways. Confining our remarks for the present to the respiratory process as it occurs iu man and mammals. we may consider the anatomical details under three different heads. First, there must be a special respiratory organ—the lungs— affording by its internal arrangement an immense extent of internal surface, covered by vascular net-work, through which the blood flows in innumerable minute streamlets, only separated by an extremely thin membrane from the atmospheric air that has been inhaled; secondly, there must be such an arrangement of the circulating system that fresh blood may be prpetually driven from the right side of the heart through the lungs, and onward to the left side of the heart; and thirdly, there must be arrangements for the fre quent and regular change of the air contained in the lungs. These three points will be considered in the order in which we have placed them.
A sufficiently large internal aeratiug surface might of course be obtained by increas ing the size of the air-bags themselves, but this would involve an increase of size in the animal. In examining the lungs of different animals, two plans-are observed for increas ing the internal surface without increasing the total bulk of the lungs. According to one plan, the internal surface is. as It were, molded into cells, separated laterally by partitions, somewhat like the cells as seen in a section of honeycomb, or more like the appearance presented by the second or honeycomb stomach of ruminating animals; according to the other, enormous multitudes of little lung-sacs partitioned, as will be presently shown, in their interior, are clustered round the ultimate branch of a common air-tube, which communicates with all of them. If we can conceive a bunch of grapes with its stem and all its minute branches, and the grapes attached to the ends of these branches completely hollow, we get a good idea of this second plan, except in so far as the partitioning of the termival cells is concerned. By the former method, which occurs in amphibians and reptiles, the lung sacs are merely rendered more cellular in their interior; while, by the latter plan, compound lungs arc formed, such as occur in birds and mammals, including man. Hence these two varieties of lung-structure correspond to the so-called cold-blooded and warm-blooded animals respectively. In the lungs of the frog and the lungs of the turtle, we have illustrations of the first plan (the cellular lung-sac), while in figs. 1 and 2 we have diagrammatic illustrations of the human lung. Fig. 1 is a shaded diagram (copied from Mr. Marshall's admirable series of Physiological .Diagrams to show the ramifications of the air-tubes in the human lungs. L is an outline representing the left lung; T, the main air-tube, called the windpipe or trachea (so called from the Greek word tracheia, rough, and similarly termed in Latin the arteria avow, although not an artery, as we now employ the word), descends through the neck from the larynx or organ of voice into the chest; B shows the right and left bronchi, or primary divisions into which the windpipe separates, one for each lung. Each bronchus enters the lung at the so
called root, and divides and subdivides into smaller branches, which never coalesce, but continue separate, like the branches and twigs of a tree. These are the bronchial tubes, or the bronchia of some writers; the smallest shown in this diagram, b, b, undergo many further subdivisions, until (to use Mr. Marshall's own description) "at length they form an immense number of minute tubes, not more than of an in. in diameter, each of which ends in a cluster of cells, or, as it may otherwise be described, opens into a small membraneous sac, a little wider than itself, having a cellular internal surface very similar to that of the frog's lung, but of course on a microscopic scale." In fig. 2 (also copied from Mr. Marshall's diagrams) there is a representation, magnified about 160 diameters, of three of these clusters of cells, or little lung-sacs, from the human lung. In this figure, b is a small air-tube, or bronchical tube, from which several of the finest or ultimate tubes proceed; c shows the outer surface of one of the lung-sacs, or lobules, as they are commonly termed; d, the inner surface of another, which has been cut open, so as to show the ultimate recesses of the lung to which the air has access—viz., the air cells. According to Rossignol, the ultimate bronchial ramifications terminate in a shape resembling that of an inverted funnel, and hence he applies the term infundibula to these endings. In fig. 3 (copied from Rossignol's Memoir) there is a representation of the termination of an ultimate bronchial tube in the lung of a dog: a represents an ultimate tube, or lobular passage, branching toward the infuudibula; b is the interior of one of the seven infuudibula shown in the figure; while c represents one of the numerous septa or partitions projecting inward on the infundibular wall, and forming the air-cells. According to Todd and Bowman, the diameter of the lobular passages is from to ,htli of an in., while that of the cells ranges from to of an inch. It is on the inner surface of these air-cells that the net-work of minute capillaries is spread in which the act of aeration takes place. Each lobule receives air through its own bron chial tube alone, and consequently there is no direct communication between the air cells of adjacent lobules. These lobules are closely compressed upon one another; and collectively, together with the connective tissue which unites them to one another, make up the great mass of the lungs. To such an extent is the process of subdivision carried out, that, according to calculation, the lungs of an adult man contain at least 600 millions of these air-cells. It is in consequence of the air included in these cells that the pulmonary tissue has a soft spongy feel, and crackles When compressed between the fingers (see RESPIRATO1CY SOUNDS); and for the same reason, the lungs, and even small portions of them, .even after. strong pressure, float in water, it being extremely difficult to drive all the air out of the cells. The lungs (except in the fetal state, when no air enters them) are thus the lightest organs, in relation to their size, in the body. Although their bulk is so great that, with the heart, they occupy almost the whole of the cavity of the chest, they only weigh about three pounds and a half in men, and two pounds and three-quarters in women. Their color varies at different ages. At birth, they are of a pinkish white tint; in adult life, they are of a slate color, and present a mottled appearance; and in old age, they become of a still darker tint. The polygonal mark ings which arc seen on the surface correspond to the outer surface of the lobules already noticed. Their adapted to that of the cavity in which they are lodged, each lung being conical in form, with its apex rising into the neck; while its base, which is broad and concave, rests upon the convex surface of the diaphragm; and between the two lungs lie the heart end the great vessels that, proceed from it. During life (except in certain diseases, as for instance, pericarditis (q.v.), the inner margins of the lungs nearly overlap the heart, leaving only a roundish space, less than 2 in. in diameter, of that organ uncovered, while their lower borders extend to the cartilages of the ribs, and fit into the angle formed between those cartilages and the diaphragm. Each lung is invested by its own serous membrane, the pleura (q.v.), which serves the double pur pose of facilitating the movements which the lungs undergo in the act of respiration, and of suspending each lung in its proper position. In the latter function, the pleurae are essentially assisted by the great air-tubes and blood-vessels, which collectively form what are termed the roots of the lungs.